On the Web:

Further information about ISDN can be found at this Harvard University web site.

Connected: An Internet Encyclopaedia provides many useful 'protocol overviews', including the following ISDN-related articles:

I.430  - BRI physical layer  
Q.921 - LAPD, the D channel link layer 
Q.931 - Connection control 
G.711 - encoding voice on the B channel 

In Print:

A particularly good (but very technical) text book on ISDN and related products is "ISDN and Broadband ISDN with Frame Relay and ATM" by William Stallings, published by Prentice Hall International Editions. Being a university student text, it is very good value for money. (ISBN 0139737448.)

Standards and Recommendations:

ITU-T standards can be bought (they are not available free) from here.

ETSI (Euro-ISDN standards) can be downloaded (free) from their web site.